1. Field of the Invention
This invention is related in general to metallic flakes of reflective, conductive and/or transparent material used in a variety of applications, such as in the manufacture of clear conductive surfaces and inks and paints for highly reflective coatings. In particular, the invention pertains to a method of producing highly reflective, passivated, nano-thick aluminum flakes from metal/organic multilayer structures deposited entirely in a vacuum environment. The multilayer deposition may be carried out either on a web using a roll-to-roll process or on a rotating drum, where thousands of aluminum layers are interleaved with specially prepared organic layers and passivated in vacuum. In either case, the aluminum layers are suitable for reduction to single flakes for the production of highly reflective pigments.
2. Description of the Related Art
Metallized films are commonly produced for a variety of applications that include decorative, packaging and low-emissivity applications for which high reflectivity is desirable. In conventional metal-flake pigment technology, such as in the production of aluminum pigments, a polymer film web is coated with a dissolvable polymer layer at atmospheric conditions and a nano-thick aluminum layer is deposited over it in a vacuum chamber. This process may be repeated several times on one or both sides of the web before the aluminum is removed from the web and reduced to a nano-flake pigment.
Polymer/aluminum multilayer structures may also be produced entirely in a vacuum chamber by depositing both materials in successive layers. Such structures have been used advantageously in the past to produce aluminum pigments in powder form where the polymer is retained with the aluminum particle (see, for example, U.S. Pat. No. 5,912,069; and A. Yializis, et al., “Low Emissivity Polymer-Metal Pigments and Coatings,” 1997 Meeting of The IRIS Group On Camouflage Concealment and Deception, Volume 1, October 1997). The same process has been utilized to manufacture multilayer capacitors (see U.S. Pat. Nos. 5,018,048, 5,125,138, and 5,731,948) wherein the highly cross-linked polymer layers are used as the dielectric between the aluminum electrode layers.
Commonly owned U.S. Pat. No. 6,270,841, herein incorporated by reference, described producing aluminum flakes by evaporating a polyethylene-oligomer in a vacuum chamber, depositing it as a solid coating (0.5 to 1.0 micron thick) on a cold polyester web, and then depositing an aluminum film in-line on top of the oligomer release coating according to conventional vacuum deposition. Aluminum metal flakes were recovered from the bulk deposition product by crushing the aluminum film within it to produce flakes, and then either by melting or dissolving the release material away from the flakes. Similar aluminum-flake products were produced using poly(α-methylstyrene) oligomers as the release layer. (Aluminum readily oxidizes into a non-conductive material; therefore, it is not suitable for manufacturing conductive layers.)
Metallic particles have also been utilized to manufacture conductive surfaces used in many modern electronic applications. Conductive plates and inks incorporate metal pigments composed of materials such as gold, silver, chromium, palladium, platinum, nickel, indium and copper. Typically, these conductive surfaces are manufactured by dispersing a metallic or metal-oxide powder in a binder and applying the mixture as a coating over a substrate. More recently, for the display industry, clear conductive inks have been developed that utilize micro- and nano-size particles of conductive oxides, such as ITO and IZO. These nano-size powders are manufactured chemically by precipitation from a solution or mechanically by grinding solid nuggets. The resulting powder particles, normally in the order of nanometers in nominal diameter, are not optically transmissive even though the material in thin-film form may be transparent. This is because the light scattering produced by reflection of dispersed random-shape particles greatly reduces the transparency of the bulk material, just as in the case of pulverized glass particles. Therefore, the application of a conductive powder to a clear substrate such as plastic or glass, while producing a conductive surface, tends to yield a translucent but not perfectly clear layer even when ITO or IZO is used.
It has been known that the metal-oxide compounds commonly used in the manufacture of conductive layers, most commonly ITO and IZO, remain transparent in flake form because of the high aspect ratio associated with the flake form (i.e., the ratio of the nominal diameter of the two-dimensional surface to the thickness of the flake). Accordingly, the use of metal-oxide particles in flake form to manufacture clear conductive surfaces is very desirable.
U.S. Ser. No. 10/355,373, herein incorporated by reference, and U.S. Pat. Nos. 6,270,841 and 6,398,999 describe a process for producing such metal flakes. A polymeric release agent is flash-evaporated and deposited onto a support substrate under conventional vacuum-deposition conditions and a pigment (i.e., aluminum) or a conductive-material precursor (e.g., ITO) is subsequently vacuum deposited onto the resulting release layer in the same process chamber to form a very thin film. The resulting multilayer product is then separated from the support substrate, crushed to brake up the film into flakes, and heated or mixed in a solvent to separate the soluble release layer from the flakes. Thus, by judiciously controlling the deposition of pigment (or conductive material) on the release layer, flakes may be obtained with the desired optical and physical characteristics.
As a result of the continuous vacuum deposition of the release layer and the pigment (or conductive material) in rapid succession on a rotating drum to form a continuous two-layer spiral of arbitrary length, the process of Ser. No. 10/355,373 allows the production of a large bulk volume of pigment (or conductive-material) film embedded between release layers, which in turn yields extensive quantities of flakes by crushing the layered product and heating or dissolving the release material.
The flakes so produced are mixed in conventional carriers and binders to make dispersions suitable for application as inks and coatings over clear substrates. Thus, the process of Ser. No. 10/355,373 produces pigment and ITO flakes without also introducing the opacity that is characteristic of the prior-art inks and coatings based on powders. Because of the higher aspect ratio of flakes with respect to powders, the conductive flakes also provide greater contact among adjacent particles and therefore also greater conductivity than is typically achievable by the use of powders.
The technology described in Ser. No. 10/355,373 permitted the manufacture of pigment and conductive flakes, rather than powders, in bulk and in a single process, with the attendant benefits associates with the single-layer flake structure. In particular, these benefits included an expected higher reflectivity or transparency than powders, as applicable (with respect to optical properties), and higher conductivity than powders with respect to applications requiring good conductivity.
Unfortunately, it was discovered that these improved properties deteriorate rapidly as soon as the flakes are utilized as pigments or conductive films. In the case of aluminum pigments, in particular, the flakes lose gloss and brightness immediately upon separation from the release material. Similarly, the conductivity of conductive films tends to decrease with time. The present invention is directed at providing a solution to these problems by perfecting the manufacturing process described in Ser. No. 10/355,373.